The present invention concerns a fastening element for use in bone, and in particular to a device for fixating implants on or in bone.
Implants have generally been anchored in the bone by means of screws, more recently also by means of monocortical screws anchored in an angularly stable manner. These screws have been designed, as far as their form is concerned, for use in healthy bone. In fact, the ratio between the cortical screws and the thickness of the corticalis has been optimized for enabling the best possible transmission of the occurring strains. These angularly stable screws no longer have the function of tension screws in the classical sense of the term but rather that of bolts. However, the screws currently in use for these purposes have dimensions which are optimized for use in healthy bone. As a consequence, the angularly stable screws, due to a reduced ratio between core diameter and outside diameter, are characterized by inferior strip forces and failure strains. A need therefore exists for an innovative means to anchor implants in bone.
The present invention is directed to a fastening element for use in bone, and in particular to a device for fixating implants on or in bone. The device may have a longitudinal axis and may comprise a core and at least one wing extending from the core. The wing may have a peripheral enlargement adapted to contact bone. A cross-section of the device transverse to the longitudinal axis may also have first and second portions, the first portion comprising the core, the second portion comprising the peripheral enlargement of the at least one wing. The second portion preferably is greater than the first portion. The ratio of the second portion to the first portion may preferably be at least about 3:1
In general, the device may have a plurality of wings and at least one of the peripheral enlargements may have a cross-section with a perimeter having at least one curved segment. The device may have peripheral enlargements which are substantially the same, and the peripheral enlargements may be arranged substantially in radial symmetry about the core. The peripheral enlargements may also be substantially point symmetric.
The core of the device may also comprise a free end having a coupling portion configured and dimensioned to join with an implant. The coupling portion may be adapted to contact a surface of the implant in positive engagement, or the coupling portion may be adapted to contact a surface of the implant in non-positive engagement. The coupling portion may also include a closing mechanism that is capable of securing the device to the implant. The closing mechanism may be capable of releasably securing the device to the implant. The implant may comprise a bone plate that may have a bore which is configured and dimensioned to correspond with the cross-section of the device to allow an angularly stable anchoring of the device in the bone plate.
The core may also be provided with a bore, which may have a cross-section that is substantially circular. For instance, the bore may have a diameter of about 2 mm. The device may also be spirally twisted. For example, the spirally twisted portion of the device may comprise a length of about 8 mm to about 12 mm, and the spirally twisted portion may be twisted relative to the longitudinal axis by about 100 to about 140 degrees.
The device may have a cross-section that has a moment of inertia about a first coordinate axis and a moment of inertia about a second coordinate axis, the first and second coordinate axes being orthogonal to the longitudinal axis of the device, and the ratio of the maximum moment of inertia to the minimum moment of inertia about the first and second axes being less than or equal to about 5:1.
The device may be a bone screw comprising a core, having a bore with a diameter and an inner surface disposed concentrically about the longitudinal axis of the screw. At least one wing may extend from the core, the at least one wing having a peripheral enlargement adapted to contact bone. The core and the at least one wing may be contained in an imaginary cylinder having a central axis coincident with the longitudinal axis. The imaginary cylinder may abut the at least one wing and may have a diameter such that a ratio of the diameter of the bore divided by the diameter of the imaginary cylinder is less than or equal to about 0.5. In one embodiment, the ratio between the diameter of the bore and the diameter of the imaginary cylinder preferably is less than or equal to about 0.30.
In general, a line segment from a point on the inner surface of the bore to a point on the imaginary cylinder may define a length equal to one-half the diameter of the imaginary cylinder minus one-half the diameter of the bore. In addition, a cross-section of the device transverse to the longitudinal axis may have an outer surface portion and an inner surface portion. The outer surface portion may be located within a first imaginary annulus having an outer diameter equal to the diameter of the imaginary cylinder, and an inner diameter equal to the length of the line segment from a point on the inner surface of the bore to a point on the imaginary cylinder. The inner surface portion may be located within a second imaginary annulus having an outer diameter equal to the inner diameter of the first imaginary annulus and an inner diameter equal to the diameter of the bore. In one example, the ratio of the outer surface portion to the inner surface portion may be at least about 3:1. In another example, the ratio of the outer surface portion to the inner surface portion may be at least about 3.5:1. In yet another example, the ratio of the outer surface portion to the inner surface portion may be at least about 4.0:1.